The subject matter disclosed and claimed herein relates to an improved method of operation of pacemakers and implantable cardioverter-defibrillators (ICDs) having pacing capabilities, which improved method serves to prolong battery longevity by deactivating certain power-consuming features while not needed and reactivating as necessary. Also provided herein are improved devices employing these methods.
The heart functions to pump life-sustaining blood throughout one""s body. The human heart comprises a left side and a right side with each side having a first chamber known as the atrium and a second chamber known as the ventricle. The right atrium receives blood from the body after the body has extracted the oxygen therefrom and the left atrium receives oxygenated blood from the lungs. At an appropriate time, an electrical stimulus is provided to the atria that causes the muscle tissue to depolarize. Immediately following depolarization, the atrial muscle tissue physically contracts, forcing the blood held in the right and left atria through one-way valves into the right and left ventricles, respectively.
The electrical stimulus provided to the atria also stimulates the ventricles after a delay which is sometimes referred to as the xe2x80x9cnatural conduction timexe2x80x9d of the heart. Upon stimulation, the ventricular muscle tissue depolarizes and then contracts. This forces the blood held within the right ventricle to pass through the pulmonary artery to the lungs and the blood held within the left ventricle to pass through the aorta to the rest of the body. In this manner, then, the heart xe2x80x9cbeatsxe2x80x9d or pumps blood by having the atria contract and, after the natural conduction time, by having the ventricles contract. After a longer delay, during which delay the right atrium is refilled with blood returning from throughout the body, the process repeats.
Implantable pacemakers and cardioverter-defibrillators (ICDs) are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more heart chambers, when necessary. For example, a pacemaker senses an arrhythmia, i.e., a disturbance in heart rhythm, and provides appropriate electrical stimulation pulses, at a controlled rate, to selected chambers of the heart in order to correct the arrhythmia and restore the proper heart rhythm. The type of arrhythmias that may be detected and corrected by pacemakers include bradycardias, which are unusually slow heart rates, and certain tachycardias, which are unusually fast heart rates.
Implantable cardioverter-defibrillators (ICDs) also detect arrhythmias and provide appropriate electrical stimulation pulses to selected chambers of the heart to correct the abnormal heart rate. In contrast to pacemakers, however, the pulses from an ICD are much stronger and less frequent. This is because ICDs are generally designed to correct fibrillations, which are rapid, unsynchronized quiverings of one or more heart chambers, and severe tachycardias, where the heart beats are very fast but coordinated. To correct such arrhythmias, an ICD delivers a low, moderate or high energy shock to the heart. In addition to functioning as a cardioverter-defibrillator, some ICDs are designed to provide pacing support to the heart. Such ICDs sense the occurrence of a cardiac arrhythmia and automatically apply an appropriate therapy to the heart aimed at terminating the specific arrhythmia detected. This type of therapy is referred to as xe2x80x9ctiered therapyxe2x80x9d.
In a tiered therapy ICD, each xe2x80x9ctierxe2x80x9d or level of therapy generally corresponds to a different type of arrhythmia and typically to a specified number of shocks of varying energies and pulse durations intended to most efficiently terminate the specific type of arrhythmia detected. Thus, such tiered therapy may include antitachycardia pacing for painless termination of monomorphic ventricular tachycardia (i.e., tachycardia that originates from one ventricular focus); programmable low-energy cardioversion also for treatment of ventricular tachycardia (e.g., when antitachycardia pacing fails to terminate the tachycardia); high-energy defibrillation for termination of ventricular fibrillation; and back-up bradycardia pacing, for ensuring the heart beats, particularly following cardioversion or defibrillation. For examples of tiered therapy ICDs, see U.S. Pat. Nos. 4,427,011; 4,541,430; 4,398,536; and 5,103,822; each of which is incorporated herein, in its entirety, by reference.
Because the invention described and claimed herein is useful in pacemakers, ICDs and tiered therapy ICDs, these devices will be collectively referred to as xe2x80x9cICD/pacemakersxe2x80x9d. It will be appreciated by those of skill in the art that discussions herein of the pacing functions of an implantable device generally refer only to pacemakers and/or ICDs having pacing capabilities, whereas discussions of cardioverting-defibrillating functions generally only refer to ICDs with or without pacing capabilities.
The pacing functions of ICD/pacemakers are described as either single-chamber or dual-chamber systems. A single-chamber ICD stimulates and senses the ventricular chamber of the heart. A dual-chamber system stimulates and/or senses in two chambers of the heart (an atrium and a ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
A three-letter code (sometimes expanded to a five letter code) is used to describe the basic mode in which the ICD/pacemaker is operating. The three-letter code concerns how the device operates to sense the need for and provide electrical stimulation to the heart. A fourth position (when used) identifies the degree of programmability and rate modulation of the device, and a fifth position (when used) refers to electrical stimulation therapy for the primary treatment of tachycardias and fibrillations.
The first position of the three letter pacemaker code identifies the chamber to which the electrical stimulus is delivered. If the device is not capable of bradycardia support pacing, an xe2x80x9cOxe2x80x9d occupies this first position. If the unit paces in the ventricle, this is identified by a xe2x80x9cVxe2x80x9d; if it paces in the atrium, the first position is identified as an xe2x80x9cAxe2x80x9d. If stimuli can be delivered to either the atrium or ventricle, the letter xe2x80x9cDxe2x80x9d is used to reflect dual-chamber stimulation.
The second position of the pacemaker code identifies the chamber or chambers in which sensing occurs. Sensing is the ability of the pacemaker to recognize the intrinsic electrical activity of the heart. The letters used in this position are identical to those used in the first position, i.e., xe2x80x9cVxe2x80x9d for ventricular sensing; xe2x80x9cAxe2x80x9d for atrial sensing; xe2x80x9cDxe2x80x9d for dual-chamber sensing; and xe2x80x9cOxe2x80x9d if no sensing capability is present.
The third position of the pacemaker code identifies the way the pacemaker responds to a sensed signal. An xe2x80x9cIxe2x80x9d means that the pacemaker will be inhibited. The inhibited mode of response indicates that when the pacemaker senses or sees an intrinsic electrical signal, it inhibits its own output pulse and resets one or more internal timers within the pacemaker""s circuitry. The other basic response is represented by a xe2x80x9cTxe2x80x9d, which means triggered. The triggered mode of response indicates that when the pacemaker senses an intrinsic electrical signal, it not only resets various internal timers within the pacemaker, it also initiates or releases a stimulus in response to that sensed event.
The most sophisticated response mode is represented by a xe2x80x9cDxe2x80x9d in the third position and refers to both modes of sensing response. Most commonly, a sensed signal arising from the atrium and sensed on the atrial channel of a dual-chamber pacemaker will inhibit the atrial output but trigger a ventricular output after a brief delay (the AV delay). If a native ventricular depolarization does not occur before the AV delay timer completes, a ventricular stimulus will be released at the end of this AV delay. If a native ventricular signal is sensed within the AV delay, the ventricular output will be inhibited and other timers will be reset. If a native ventricular signal is sensed before the atrial stimulus is released, both the atrial and ventricular output pulses will be inhibited and the various timers will be reset.
In order to perform their pacing and/or cardioverting-defibrillating functions, pacemakers and ICDs must have an energy source, e.g., a battery. Because replacement of the battery requires explantation of the device and implantation of a new device, it is critical that the battery last as long as possible. A popular mode of operation for dual-chamber pacemakers, and, increasingly, ICDs having pacing capabilities, is the xe2x80x9cDDDxe2x80x9d mode. In this mode, the device provides electrical stimuli to both an atrium and a ventricle of the heart, senses electrical activity in both the atrium and ventricle and provides both inhibited and triggered responses to sensed electrical activity. Operating an ICD or pacemaker in the DDD mode consumes more battery energy than a mode of only single-chamber sensing and/or stimulation. For example, it is estimated that an ICD that also provides DDD pacing consumes roughly 20% more battery current than one that provides VVI pacing only. This translates into a reduction in longevity for the typical ICD of about one year.
ICD/pacemakers having DDD pacing capability are advantageous in that they are able to provide more appropriate therapy to the patient than such devices operating in, for example, VVI mode where stimulation and sensing occur only in the ventricle and the mode of response is only by inhibition of the pacing signal. Although VVI pacing may be usually sufficient for the patient, having DDD pacing available provides more complete therapy to the patient. Thus, what is needed are pacemakers and ICDs capable of providing DDD pacing without such a dramatic increase in energy consumption.
The subject matter disclosed and claimed herein advantageously addresses the above and other needs by providing an improved method of operation for ICD/pacemakers, which method provides for battery conservation in DDD pacing devices. Further provided herein are improved ICD/pacemakers employing the improved method.
Generally, the improved method provides for the deactivation of one or more power-consuming features of the implantable medical device during periods when those features are not critical or are not needed. For example, most ICD patients infrequently require pacing support. Further, when pacing support is required, it is usually needed only for a short time. Thus, in a most preferred embodiment, the improved method contemplates deactivation of the atrial sense amplifier during normal operation of an ICD, leaving active only the ventricular sense amplifier and pacing circuitry. Upon the occurrence of predefined events, for example excessive sequential ventricular pacing, the atrial sense amplifier is activated, and DDD pacing, if appropriate, is initiated. The atrial sense amplifier then remains activated until one or more predefined events occur, such as a return to a ventricular rate above the brady limit, at which point the atrial sense amplifier is again deactivated.
In an alternative embodiment, an improved method of operation of an ICD is contemplated, which method includes activation of the atrial sense amplifier and initiation of DDD pacing for a predefined period of time immediately following the administration of cardioversion and/or defibrillation therapy by the ICD. Since administration of cardioversion-defibrillation therapy can give rise to a bradycardia episode, this improved mode of operation is advantageous.
With respect to patients requiring only pacing support, rather than cardioversion-defibrillation and pacing, the improved method may be employed in DDD pacing pacemakers, thereby extending the useful life of those devices. For example, some pacemaker patients are amenable to single-chamber pacing support, but may benefit, from time to time, from dual-chamber pacing support. The general method is the same, that is, the atrial sensor remains deactivated until predefined cardiac events, sensed by the ventricular sense amplifier, suggest a need for dual-chamber pacing. At this point, the atrial sense amplifier is activated, and, if appropriate, a DDD mode of pacing is begun. Once the patient""s heart rate has returned to within predefined acceptable limits, the atrial sense amplifier is again deactivated. As will be appreciated by those of skill in the art, an xe2x80x9cacceptable heart ratexe2x80x9d may be determined in numerous ways, for example, a return to a rate above the patient""s brady limit or recordation of a substantial number of P-waves followed by R-waves are indicative of a stable heart rate that no longer requires DDD pacing. Similarly, examples of events that may be monitored for the purpose of determining when DDD pacing may be necessary include an excessive frequency of ventricular pacing and/or an excessive number of sequential ventricular pacing events. Thus, using the improved method described and claimed herein, these pacemaker patients may advantageously use their implanted device for a longer period of time.
Also provided herein are implantable ICDs and pacemakers employing these new methods to extend the useful lives thereof. In preferred embodiments, these devices include means for generating stimulation pulses; first sensing means for sensing electrical activation of the heart at the right and/or left ventricle; second sensing means for sensing electrical activation of the heart at the right and/or left atrium; activation/deactivation means, responsive to the first and/or second sensing means, for activating and deactivating the second sensing means and/or the means for generating stimulation pulses; microprocessor control and timing circuits; a clock; and a power source, such as a battery. In a most preferred embodiment, the implantable device further includes memory for storing information on the operation of the device as well as patient-specific information and data; and telemetry means for receiving/sending information and data from/to the memory and from/to an external programmer. Additionally, if the implantable device is an ICD, cardioverter-defibrillator circuitry is included.
Thus, it is a feature of the present invention to provide an implantable medical stimulating device, and method of operating such device, which conserves the limited energy of the device""s battery.
It is a another feature of the invention to provide an implantable medical device that can operate in at least two modes, and wherein the device automatically operates in whichever mode consumes the least power, except when conditions require operation in another mode.